The idea that the American military leaves none of its soldiers behind has been under perhaps more scrutiny than ever since May 31.

That's the day Sgt. Bowe Bergdahl was exchanged with the Taliban for five Taliban captives held by the U.S. at the prison on Guantanamo Bay, Cuba. Bergdahl had been imprisoned by the Taliban since 2009, and was the last American prisoner of war.

Since the transfer, many have questioned the circumstances surrounding his capture and release. Some say he deserted his post and have openly questioned whether the American military should have devoted resources and other soldiers to saving such a person. Others have wondered whether the exchange price—five Taliban prisoners—was too high.

But the military's response, from President Obama and Secretary of Defense Chuck Hagel on down and regardless of whether Bergdahl abandoned his duties, has been simple: America does not leave its soldiers behind.

The nature of that statement is cut and dry, yet its certainty doesn't always hold up when viewed through the lens of history. Though the concept dates to before the country was founded, it has evolved with military technology and changed as America's armed forces stopped employing a draft and became a volunteer organization. And sometimes, when the military has deemed it necessary, the concept has been abandoned altogether.

“I think there’s plenty of evidence that we’ve [left soldiers behind]," Thomas Phillips, coauthor of the book Leave No Man Behind: The Saga of Combat Search and Rescue, told Mashable. "We’re not perfect.”

In fact, it's not even clear where the mantra came from.

The French and Indian War

If you're looking for the start of that ethos in the American military, your best best might be to start with the French and Indian War that began in 1756, 20 years before the United States declared independence from the British.

During that war, a group of American soldiers known as Rogers' Rangers fought for the British against the French, using a combination of pioneer techniques and native-American tactics to outsmart enemy soldiers in wooded terrain where traditional militias struggled. They were also known for holding a certain standard, according to Paul Springer, an associate professor of comparative military studies at the Air Command and Staff College, which was to leave no fellow soldier behind.

“This has been a part of American society before there was an American society," Springer, told Mashable. “There was a certain mentality of no man left behind.”

But that was a hard ideal to put into practice without technology that made search-and-rescue missions feasible. How were soldiers before planes or surveillance technology supposed to track anyone taken captive? How would they fight through enemy soldiers to get the prisoners home?

The Civil War

At times, government decrees clashed with "no man left behind" in a way that put the military concept on hold.

During the Civil War, a conflict during which Phillips said the Union military made several attempts to rescue thousands of its captured soldiers, the North began to enlist African American service members.

The Confederacy was horrified by the decision, according to Phillips, and declared that it would enslave any black soldiers its units captured.

In response, the Union said it would halt all prisoner exchanges until the South agreed to treat captured black soldiers the same as whites. The Confederate States refused, and around 13,000 Union soldiers in Confederate custody died instead of being traded and sent home, according to Phillips.

The World Wars

World War I brought about a change in military search-and-rescue technology with the advent of planes.

Being able to see from the sky allowed airmen to search for lost soldiers, yet World War I and World War II did not prove to be ideal conflicts in which to rescue soldiers who had found themselves in danger behind enemy lines. The opposing militaries were often too powerful to risk many operations into enemy territory for a few soldiers.

“There are limits to the concept of what we will and won’t do," Springer said.

At the end of World War II, when the U.S. was deciding whether to detonate atomic bombs in the Japanese cities of Hiroshima and Nagasaki, the government was aware that American POWs were being held in those cities, according to Springer. Ultimately, the U.S. decided to drop an atomic bomb on each of the above cities, knowing full well that Americans were likely to die in the process.

Yet, despite the pragmatism that provided the basis for the decision to drop the atomic bomb, World War II saw some successful attempts to rescue captured American service members that weren't practical at all.

During The Raid at Cabanatuan in the Philippines, also known as The Great Raid, the U.S. military sent soldiers into the teeth of Japanese forces tofree around 500 U.S. prisoners of war in what experts said is still the biggest rescue attempt in history.

"It was a huge risk," Springer said. "We could have lost everybody involved."

The Vietnam War

Planes altered the practicality of not leaving a soldier behind, but their impact on the ethos was arguably no greater than America's switch to an all-volunteer military toward the end of the Vietnam War.

“When you have a conscript army and you can always replenish it just by adding more people, you don’t really have to care about whether they’re happy with what they’re doing," Springer said.

Now the military had to care about its soldiers as individuals, and the idea that it would never leave them behind became something of a familial bond.

“It’s kind of a contract with the service," Springer said. "You promise to serve us, we promise not to leave you.”

In the midst of the Vietnam War in 1974, Army Chief of Staff General Creighton Abrams created battalions called Rangers, elite infantry soldiers derived from the Rogers' Rangers of the French and Indian War. He also developed the Ranger Creed, which says "I will never leave a fallen comrade to fall into the hands of the enemy..." Other divisions of the military have similar codes of conduct.

The Modern Media Era

At the same time the "no man left behind" concept became official, Phillips said it also started to get more attention from the public. U.S. military deaths weren't uncommon, but they were becoming infrequent enough that the loss of an individual's life was news. And so, when an American soldier was lost or left behind, the public knew about it.

Hollywood also helped build awareness of the notion that the U.S. goes back for its lost soldiers. The 2001 movie Black Hawk Down immortalized the rescue of Michael Durant, the chief warrant officer who was rescued from Somali militants after 11 days in 1993, and films such as The Great Raid have honored other success stories.

The Black Hawk Down incident was a hostage rescue, while The Great Raid was an attempt to free prisoners of war. Though the two are distinct, The Great Raid was conducted because the lives of prisoners at Cabanatuan were thought to be in danger, just like Durant's life was in Somalia. According to U.S. Secretary of Defense Chuck Hagel, the American military decided to swap Taliban prisoners for Bergdahl in part because a video of Bergdahl from January, 2014 showed him to be deteriorating mentally and physically.

On top of that, Bergdahl's situation was arguably different from The Great Raid and the rescue in Somalia. In a way, it combined elements of both. He was a prisoner of war, like those being held at Cabanatuan, but also a captive being held by a stateless group, much like the situation in Somalia. It's worth noting that America is still at war with al-Qaeda, a rogue terrorist organization with many offshoot organizations but no firm territory.

The experts Mashable spoke with can't remember a scandal surrounding the retrieval of an American soldier prior to Bergdahl's release, and certainly not anything like the heated political debate focused on the circumstances of his transfer.

But the military seems likely to carry out its ethos no matter what comes of the current furor.

Far away from Washington, D.C., on the island of Oahu in Hawaii, resides a military unit called the Joint POW/MIA Accounting Command. Every year, it uses a budget of around $55 million not for search and rescue missions, but to find and collect the remains of soldiers who have died in wars that have long been over.

Even in death, the idea of "no man left behind" is there.

“That’s always been a part of the ethos," George Galdorisi, Phillips' coauthor of Leave No Man Behind: The Saga of Combat Search and Rescue, told Mashable. "I’m not going to leave my buddy on the battlefield if he’s wounded, I’m going to try not to leave him on the battlefield if he’s dead.”

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